Nanotechnology Spotlight

(Nanowerk News) The discovery of numerous nanomaterials has added a new dimension to the rapid development of nanotechnology. Consequently, the professional and public exposure to nanomaterials is supposed to increase dramatically in the coming years. Especially, carbon-based nanomaterials (CBNs) are currently considered to be one of the key elements in nanotechnology. Their potential applications range from biomedicine through nanoelectronics to mechanical engineering. Thus, it is primordial to know the health hazards related to their exposure. As the public calls for safety studies get louder more and more researchers begin to study the potential toxicity of nanomaterials. Especially carbon-based nanomaterials, due to their numerous and wide-ranging applications and increasing real life usage, get nanotoxicological attention. Scientists in Switzerland studied the toxicity of carbon- based nanomaterials (nanotubes, nanofibers and nanowires) as a function of their aspect ratio and surface chemistry. Their work clearly indicates that these materials are toxic while the hazardous effect is size-dependent.

Quite a number of members of the scientific community are concerned about the toxicity of carbon nanotubes because of their structural resemblance to asbestos.

Chrysotile, which constitutes up to 95% of all industrial asbestos, is a basic silicate of magnesium, a naturally occurring mineral that is not toxic. The chrysotile structure is a sheet that bends to form tubes, which give the mineral the fibrous habit related to asbestos. The problem is that one large asbestos fiber can become the source of many smaller fibers over the course of time. As they get progressively smaller (as thin as 10 nm) and lighter, they become more mobile and more easily airborne, often resulting in human respiratory exposure. The small fibers do not decompose or degrade. Exposure to these broken-down tubular forms is what causes a health hazard that kills thousands of people every year.

In the case of asbestos, where a benign silicate mineral becomes carcinogenic in its fibrous form, the size, aspect ratio, and surface charges have proven to have a strong influence on their toxicity. How these parameters affect the biotoxicity of carbon-based nanomaterials is totally unknown.

In the last five years, the question about potential toxicity of carbon nanotubes was raised steadily. Although the Swiss study ("Cellular Toxicity of Carbon-Based Nanomaterials") shows a lower toxicity of carbon nanotubes compared to carbon black or filaments, precautions in their manipulation need to be taken. In particular, in applications where carbon nanotubes are injected into human body for drug delivery, for example, as contrast agent carrying entities for MRI, the toxicity issue must be carefully addressed.

Cytopathological analyses of H596 cells (a) A typical control image of H596 cells is depicted; cells were stained with hematoxylineeosine; the nuclei appear purple and patchy; the cytoplasm is weekly stained (pink). Clusters of cells are characterized by close cell/cell contacts, and individual cells are polygonal-shaped. (b) H596 cells after 1 day treatment with 0.02 µg/mL of MWCNT. Cells have lost their
mutual attachments, retracted their cytoplasm (arrows) such that the pink color appears stronger, and the nuclei are smaller and more condensed (picnotic) also evidenced by the stronger purple staining. (Reprinted with permission from American Chemical Society)

Magrez, a researcher at the NN Research Group (Laboratoire de nanostructures et nouveaux matériaux électroniques) at the Ecole Polytechnique Fédérale de Lausanne (EPFL) in Switzerland, led the team that investigated how shape, size and surface properties of CBNs affect cellular toxicity.

The effect on the cell proliferation and cytotoxicity of the CBNs was evaluated by the widely established mitochanodrial function (MTT) assay. The researchers added increasing concentrations of multi-walled carbon nanotubes (MWCNT), carbon nanofibers (CNF), or carbon nanoparticles to three different types of cultured human lung tumor cells (H596, H446, and Calu-1) and measured changes in cell proliferation and overall cellular health. The researchers found evidence of toxicity as soon as 24 hours after dosing with all three materials and in each cell line, though MWCNTs were the least toxic in all assays.

Magrez noted that they were surprised that carbon nanoparticles proved to be the most toxic of the three materials they studied, though he added that this finding suggests that "dangling bonds" could be responsible for the toxicity of CBNs. Highly reactive dangling bonds – carbon atoms not bonded to three other carbon atoms and thus available to react with biomolecules – are present in carbon black with a high density, whereas in carbon nanotubes they preferentially occur at lattice defects or end caps.

To explore the effect of surface chemical properties on the toxicity, the researchers performed another set of experiments in which they modified the surface chemistry of the filaments by adding carbonyl (C=O), carboxyl (COOH), and/or hydroxyl (OH) groups onto the nanotube and nanofiber surfaces. When the number of viable cells is compared after the treatment with CBNs, it became evident that the toxicity increases with the chemical surface treatment. This is significant in the case of MWCNTs and moderate for CNFs.

The latter observation might be somewhat obscured by the fact that a relatively high toxicity already occurred with unmodified CNFs. Nevertheless, these results clearly demonstrate that grafting additional putatively 'toxic' chemical groups on the surface of MWCNTs reduces the number of viable cells significantly.

The exact mechanisms that lead to cell death are still unclear, but CBNs can induce cell death either after contact with cell membranes or after their internalization.

"One has to consider that the toxicity of the investigated CBNs could be specific to the processing methods applied" Magrez points out. "Moreover, it has to be emphasized that this study does not address the carcinogenicity of CBNs, that is, the potential to transform a normal cell to a tumor cell, which requires a detailed follow-up investigation on that specific topic."